1. Field of the Invention
The invention relates to a glass film with high edge strength, to a method for producing such a glass film, and to the use of a glass film exhibiting high edge strength.
2. Description of Related Art
Glass, especially thin glass, is increasingly employed in numerous application fields, such as consumer electronics. Due to the inherently very low thickness of a thin glass of less than 1 mm, preferably in a range from 5 μm to 250 μm, such glass has a number of advantages in terms of processing. For example, it is so flexible that it can be coiled and can thus be used in roll-to-roll processes that are preferred in the field of consumer electronics. Further advantages include the high chemical resistance of thin glass, its low density, low electrical and thermal conductivity, high temperature stability, and its optical transparency which allows processing by means of lasers, for example.
Although for the aforementioned reasons thin glass is an attractive material, it however exhibits the serious drawback of mechanical instability. This instability is in particular caused by the occurrence of microcracks at the edges of the glass, and such microcracks occur when the glass is cut to the desired size and in particular when severing the glass border that is produced during hot-forming in the drawing process usually employed for manufacturing thin glass.
Another way of producing thin glass is the so-called redrawing process which is particularly attractive for the production of small batches, special glasses, or special dimensions. In this case, a preformed glass body (preform) is heated, along a line, to temperatures at which it can be deformed, i.e. at which the glass has a viscosity of less than 109 dPa·s, and the heated portion is withdrawn by a drawing mechanism. In this manner, a glass body is produced that is thinner and narrower than before, however, a planar quality portion and a thickened border are produced in this case as well. Thus, microcracks and defects which critically lower glass strength will naturally be induced in the edge region even in this case, when the border is severed.
Several methods have been used for improving edge strength. In case of greater thicknesses of the glass body, for example in the millimeter range, the edge may be further processed, e.g. by faceting, grinding, or polishing, but this is impossible for thin glass due to the small glass thickness thereof. Another way of edge finishing is to fill the microcracks by a coating. Moreover, special cutting methods such as by means of a laser are being discussed.
There are also discussions about the formation of a special high quality edge directly during the shaping process, for example during redrawing. For example, US 2011/0059296 A1 discloses a method for redrawing glass, in which a ribbon-shaped glass body with specially formed edge is obtained. The redrawing of the glass to obtain a narrow glass ribbon is performed in a manner so that a glass body is produced, which has rounded, i.e. convex, edges which are completely fire-polished and accordingly do not have microcracks. The method described in US 2011/0059296 A1 has a number of drawbacks.
For example, very special preforms have to be used to obtain a glass ribbon with a special edge shape. The preforms employed have widths of only 50 to 100 mm. Furthermore, it is advantageous for the method described therein, that the edges of the preform have already been processed, for example ground to take the form of a so-called C-cut so that the edges have a completely rounded C-shaped configuration. The ribbon-shaped glass body has a thickness of 100 μm and below, and the aspect ratio between width of the ribbon and thickness thereof is from 25 to 2000 is, so that ribbons with a maximum width of 200 mm can be obtained in this case.
US 2012/0070618 A1 discloses the use of such glass ribbons with rounded fire-polished edges as a sealing material.
Another method for producing a glass article with specially formed edge is disclosed in published patent application DE 10 2011 084 128 A1. Here, the special form of the edge of a thin glass sheet is achieved by introducing energy into the glass along the intended separation line using a laser beam, wherein prior to the severing the glass has a working temperature of at least more than 250 K below Tg, the transformation point of the glass. With this method described in DE 10 2011 084 128 A1, the edge of the thin glass obtained has fire-polished upper and lower surfaces. It is also possible to sever thickened edge portions, so-called borders, that are produced during the manufacturing process of the glass. However, formation of a bead is caused in this case, i.e. a thickening of the edges.
From DE 10 2009 008 292 B4 it is known to divide a thin glass into individual ribbons, by laser cutting. However, DE 10 2009 008 292 B4 does not include any statements about the edge quality or possible occurrence of separation beads.
Moreover, there are discussions in the prior art about so-called laser scribing. Here, the glass is first heated by irradiating the glass along a predetermined line by means of a laser beam, and then great mechanical stress is produced by immediately cooling the glass, so that the glass breaks easily along this line. Such a method is described in DE 693 04 194 T2, EP 0872 303 B1, and U.S. Pat. No. 6,407,360 B1, for example.
However, in the prior art lasers are not only used to produce edges of glass films and/or glass ribbons that are preferably free of cracks, but can also be used to selectively modify the thickness of a glass film or glass ribbon.
For example, from DE 101 28 636 C1 a method is known in which the glass thickness of a drawn or floated glass ribbon is influenced by local supply of heat by means of a laser. The method described in DE 101 28 636 C1 is in particular intended to equalize the glass thickness across the entire width, in particular it is intended to equalize, or avoid already during manufacturing, local thickness variations of the glass ribbon, e.g. fine corrugations (or so-called waviness) or warping. However, the edges quality of the obtained glass ribbon is not affected in this case.
DE 10 2008 063 554 A1 also describes a method for modifying the thickness of a flat glass, for example by means of a laser. Here, again, nothing is mentioned about the creation of a preferably microcrack-free surface of the edge.
US 2014/0123703 A1 describes a method for thickness control of a substrate, e.g. a glass substrate, which may take the form of a glass ribbon, inter alia. Again, the creation of a preferably microcrack-free surface is not mentioned.
According to the prior art it is thus possible to produce glass films or glass ribbons that exhibit high edge quality, for example by providing fire-polished surfaces, and it is possible to use lasers to selectively influence the thickness of a glass film or glass ribbon. However, for the case of specially shaped edges of a glass film or glass ribbon, complex processing steps are still necessary, such as the manufacturing of precise preforms, or separation beads are obtained.
Therefore, there is a need for an improved method for cost-efficiently producing large sheet-like glass articles or glass films that have a small thickness and improved edge shape, in particular without developing separation beads, in order to avoid microcracks and other glass defects in the edge region.